Electrostatic transducer

ABSTRACT

A method of permanently polarizing a vibratile film of an electrostatic transducer wherein an unpolarized film having a vibratile electrode on one side is disposed in a housing opposite a stationary electrode. A saturation polarizing voltage is applied across the vibratile and stationary electrodes to cause a ferroelectric phenomenon due to absorbed surface charges to occur in the vibratile film thereby permanently polarizing the vibratile film.

United States Patent 1 Yoshimura et al.

[ ELECTROSTATIC TRANSDUCER [75] Inventors: Hisashi Yoshimura, Minoo;

Nobuhisa Atoji, Shijonawate, both of Japan Matsushita ElectricIndustrial Co., Ltd., Osaka, Japan [22] Filed: Mar. 5, 1973 [21] Appl.No.: 337,843

Related US. Application Data [63] Continuation of Ser. No. 95,856, Dec.7, 1970,

abandoned.

[73] Assignee:

[30] Foreign Application Priority Data Dec. 11, 1969 Japan 44-99939 [52]US. Cl. 29/594, 29/592, 179/100.4l B, 179/111 E, 307/88 ET, 317/262 A[51] Int. Cl H04r 31/00 [58] Field of'Search 29/592, 594; 317/262;307/88 ET; 179/111 E, 100.4 B, DIG. 10

[ Jan. 7, 1975 [56] References Cited UNITED STATES PATENTS 3,705,31212/1972 Sessler et al. 307/88 ET Primary Examiner-C. W. Lanham AssistantExaminer-Joseph A. Walkowski Attorney, Agent, or Firm-Stevens, Davis,Miller & Mosher [5 7] ABSTRACT Patented Jan. 7, 1975 3,858,307

2 Sheets-Sheet 2 -/000 -600f 00 200 1600 /000 5PM I-IOU 1 ELECTROSTATICTRANSDUCER This is a continuation of application Ser. No. 95,856, filedDec. 7, 1970.

This invention relates to electrostatic type transducers andparticularly to a method of polarizing vibratile films of electrostatictype transducers, the vibratile films being permanently polarized afterassembly into housings of the transducers.

The electrostatic type speaker, which is a typical example of theelectrostatic type transducer, is usually of a construction as shown inFIG. 1 of the accompanying drawing. It comprises a vibratile film 1 of adielectric material, a vibratile electrode 2 formed by depositing orapplying a thin film of such material as aluminum and silver on onesurface of the dielectric film l and vibrating therewith, and astationary electrode 3 of a punched or perforate metal plate extendingparallel to the vibratile film 1 to form a capacitor therewith. Thestationary electrode 3 and vibrating film 1 are spaced apart a certaindistance conforming to the required acoustic characteristics of thespeaker by a spacer 5 defining an air layer 6 between them.

In the speaker of the above construction, when a-c signal e issuperimposed upon a high d-c bias voltage E between both the electrodes2 and 3, the vibratile film 1 is vibrated to produce sound pressure.

Since the prior-art transducer of the above type requires a high d-cbias voltage E for its operation, the speaker unit should include a highvoltage generator and an amplifier as well as requiring a signal inputterminal and a bias voltage terminal, which imposes restriction on thespeaker design and leads to increased manufacturing cost. Besides, thehigh d-c bias voltage will cause ionization of the air layer between theelectrodes, thus rendering the speaker sensitivity unstable resulting invarious practical problems. Further, the use of high d-c voltage isquite hazardous to the user.

To overcome the above drawbacks inherent in the electrostatic typetransducer requiring a high d-c bias voltage, it has been proposed touse the Electret film of a plastic material, which is pre-polarized andcarries a permanent electostatic charge, as the vibratile film of thetransducer. To pre-polarize the plastic film, however, it is usuallynecessary to heat the film by placing it in a strong d-c electric fieldup to its softening point, maintaining it at this temperature for acertain period of time and then cooling it down to room temperature in acertain period of time, thereby to freeze or thermoset its polarization.However, this requires not only special equipment but also complicatedmanufacturing steps, and a long time (1 to 0.5 hours) so that theproductivity is inferior and the manufacturing cost is high.

Accordingly, an object of the invention is to provide an electrostatictype transducer, which is free from the drawbacks inherent in theconventional transducers of these types and can be readily manufactured.

Another object of the invention is to provide an electro-statictransducer for use in speakers, microphones, pickups and the like, whichuses a vibratile film of a dielectric material such as a plastic filmcapable of readily undergoing permanent polarization by the forcedinduction and charge attraction phenomena therein and does not requireany d-c bias voltage for operation.

According to the invention, a permanent electrostatic charge can bebuilt up on the vibratile film by applying a saturation polarizingvoltage thereto for several seconds, so that it is possible tomanufacture an electrostatic transducer without requiring any d-c biasvoltage for its operation. Thus, it is possible to provide anelectrostatic transducer requiring no high voltage generator nor anybias voltage terminal. Also, the ionization of the air layer between theelectrodes can be eliminated to greatly improve the sensitivity andother characteristics of the transducer.

Further, the saturation polarizing voltage maybe applied after thevibrating film is assembled into a housing of the transducer such as aspeaker, microphone and the like. By so doing, the initially built-upcharge may be prevented from being otherwise neutralized during theassemblage.

The above and other objects, features and advantages of the inventionwill become more apparent from the following description with referenceto the accompanying, drawing, in which;

FIG. 1 is a schematic representation of a prior-art electrostatictransducer;

FIG. 2 is a schematic representation of the principles underlying theinvention; and

FIG. 3 is a graph representing a hysteresis characteristic showing therelationship between the polarizing voltage E applied on the vibratingfilm and the residual sound pressure after the removal of the polarizingvoltage E,,, which is obtained when the vibratile film according to theinvention is applied to a speaker.

Aluminum was deposited on one surface of an unpolarized film ofpolyethylene terephthalate known commercially by the trademark Tetron"and assembled, as a vibratile film, into a housing of an electrostaticspeaker, opposite to a stationary electrode and spaced therefrom so asto define an air layer. It was found that a capacitor consisting of thedeposited aluminum layer, Tetron film, air layer and the stationaryelectrode exhibits a hysteresis phenomenon due to absorbed surfacecharges. This phenomenon is thought to correspond to a ferroelectricphenomenon of a ferroelectric substance and is called a hysteresis ofthe condenser speaker with absorbed surface charges.

Referring now to FIG. 2, which shows the principles underlying theinvention, reference numeral 7 designates a vibratile dielectric thinfilm made of such plastic material as Tetron and Mylar trademarks forpolyethylene terephthalate, polymide or a fluorine compound resin, forinstance ethylene tetrafluoride'or polyvinylidene fluoride, capable ofreadily undergoing permanent polarization. It should be noted that thevibratile film initially undergoes no polarization. The vibratile filmis provided on one side with a vibratile electrode 8 as in theconventional transducer. It faces a stationary electrode 9 formed with aplurality of small pores 10. The stationary electrode 9 and thevibrating film 7 define an air layer 11 therebetween. Thus, a capacitorconsisting of the electrodes 8 and 9 in between which the vibratile film7 and the air layer 11 are sandwiched is formed.

With the above construction, a saturation polarizing voltage E, isapplied between the electrodes 8 and 9 such that the residual charge onthe vibratile film 7 is maximum, thereby causing the ferroelectricphenomenon in the vibratile film 7 due to absorbed surface charges andthereby polarizing permanently the vibratile film 7. The direction ofpolarization when the polarizing voltage E is applied between theelectrodes 8 and 9 is shown in FIG. 2. Even when the polarizing voltageE is removed, the vibratile film 7 still remains polarized. This isthought to be due to spontaneous polarization for permanent superficialcharge attraction. That is, spontaneous polarization is induced in thevibratile film 7 and permanent superficial charge attractionconsequently occurs in opposite surfaces of the vibratile film 7.

FIG. 3 shows a hysteresis of the output residual sound pressure p, whichis obtained by using a Tetron film 15 microns thick as the vibratilefilm 7 and applying input signal e after the application of polarizingvoltage E for several seconds without heating the vibratile film 7.

If the plastic film is absolutely non-polarized, the sound pressurewould follow the dashed plot in FIG. 3. Actually, the plastic filmusually bears a slight negative charge. Due to the natural charging ofthe film and the application of the polarizing voltage, the output soundpressure will start to follow segment 1 of the plot and then followthrough segments 2, 3, 4, 5 and 6 of the hysteresis curve in thementioned order back to segment 2 as the polarizing voltage on thenon-polarized film is increased in small steps to a positive maximum,then reduced to a negative maximum and back to the positive maximum.

When the transducer according to the invention is applied to a speaker,the saturation polarizing voltage E,,, which may be applied for severalseconds, is determined by the speaker construction, the thickness of theair layer 11 and the material of the vibratile film 7. After removal ofthe saturation polarizing voltage a residual sound pressure of about 98db may be obtained, so that the bias voltage becomes unnecessary.

What is claimed is:

l. A method of polarizing a vibratile film of an electrostatictransducer, the vibratile film being permanently polarized afterassembly into a housing of the transducer, comprising the steps of:

preparing an unpolarized vibratile film of a plastic material capable ofreadily undergoing permanent polarization,

depositing a vibratile electrode of metal on one surface of saidvibratile film, said vibratile electrode being vibratile with saidvibratile film,

assembling into a housing of the transducer said vibratile film on whichsaid vibratile electrode is deposited,

disposing in said housing a stationary electrode of metal opposite tosaid vibratile film and spaced therefrom so as to define an air layer,and

then applying across said vibratile and stationary electrodes asaturation polarizing voltage without heating said vibratile film, saidsaturation polarizing voltage causing ferroelectric phenomena in saidvibratile film due to absorbed surface changes to maximize the residualcharge on said vibratile film after the removal of said saturationpolarizing voltage, thereby polarizing permanently said vibratile film.

2. A method according to claim 1, wherein the plastic material isselected from a group consisting of polyethylene terephthalate, apolyimide resin and fluorine compound resin.

3. A method according to claim I, wherein the saturation polarizingvoltage is applied for several seconds.

1. A method of polarizing a vibratile film of an electrostatictransducer, the vibratile film being permanently polarized afterassembly into a housing of the transducer, comprising the steps of:preparing an unpolarized vibratile film of a plastic material capable ofreadily undergoing permanent polarization, depositing a vibratileelectrode of metal on one surface of said vibratile film, said vibratileelectrode being vibratile with said vibratile film, assembling into ahousing of the transducer said vibratile film on which said vibratileelectrode is deposited, disposing in said housing a stationary electrodeof metal opposite to said vibratile film and spaced therefrom so as todefine an air layer, and then applying across said vibratile andstationary electrodes a saturation polarizing voltage without heatingsaid vibratile film, said saturation polarizing voltage causingferroelectric phenomena in said vibratile film due to absorbed surfacechanges to maximize the residual charge on said vibratile film after theremoval of said saturation polarizing voltage, thereby polarizingpermanently said vibratile film.
 2. A method according to claim 1,wherein the plastic material is selected from a group consisting ofpolyethylene terephthalate, a polyimide resin and fluorine compoundresin.
 3. A method according to claim 1, wherein the saturationpolarizing voltage is applied for several seconds.